This invention relates to measurements of the concentration of carbon dioxide and of water vapor using infrared light.
In one class of instruments for measuring the concentration of carbon dioxide, infrared light is transmitted through a mixture of gases and the amount of infrared light in the high absorbance region of carbon dioxide that is absorbed is utilized to determine the amount of carbon dioxide that is present. In other systems, infrared light is transmitted through water vapor to determine the concentration of water vapor in accordance with the amount of infrared light that is absorbed by water vapor.
In the prior art, separate measurements have been made for water vapor and for carbon dioxide and usually these measurements have removed water vapor before measuring carbon dioxide. Moreover, in the prior art instruments, there are no corrections for measuring carbon dioxide to accommodate changes in the absorbance characteristics of carbon dioxide from secondary molecular level effects caused by the presence of some water vapor or other foreign gases. The prior art instruments did not correct for band broadening caused by second order effects of foreign gas on the gas being measured.
In one prior art carbon dioxide measuring instrument, infrared light having a center point at about 4.2 microns is transmitted through a chopper into a cell for measuring the concentration of carbon dioxide by determining the amount of infrared light in the absorbance band of carbon dioxide that is absorbed in the cell after both moisture and carbon dioxide have been removed from the gas to provide a zero reading. Next, the moisture is removed but not the carbon dioxide and the measurement is made again so that a comparison between the two readings provides a measurement of carbon dioxide related to the absorbance of infrared light. This type of prior art device is described in U.S. Pat. No. 4,803,370.
A modification of this type of device was made with the intention that it be used to measure both water vapor and carbon dioxide. An advertisement of such an instrument was circulated to some potential customers before it was built to determine pubic interest. However, it was not possible at that time to successfully build such an instrument partly because of the inability to correct errors related to second order effects of foreign gas in existing equipment.
The prior art instruments have several disadvantages such as: (1) only measuring one of carbon dioxide or water vapor at a time; or (2) not providing sufficient precision of measurements of both carbon dioxide and water vapor with infrared light passed through the mixture; (3) not using a sufficient proportion of the light from beam splitters that divide the light; and (4) not correcting for certain second order effects of foreign gases nor for self band broadening.